1. Field of the Invention
The present patent application relates to the field of imaging the characteristics of an object, and particularly to a measuring system allowing for detecting defects of an object by imaging the characteristics of the object having at least a first and a second layer as well as a method for such detection.
2. Description of the Related Art
Quality control is very important in for instance the packaging industry. Most manufacturers perform a manual visual inspection of the products, i.e. examining the products with the human eye. This is time consuming, expensive, not very efficient and may be considered to be a subjective evaluation. An alternative to the manual quality inspection is to use an automated inspection system. However, the inspection of transparent or semi-transparent materials is difficult since defects on these materials are invisible to normal inspection systems.
One method of inspecting transparent objects, such as glass or plastics, is to use a bright-field/dark-field system. The bright-field is the region where specular reflected light from the object is imaged on a sensor. The dark-field is the region in which diffusely reflected light from the object is imaged on a sensor. The signals from the dark- and bright-field sensors can be used to detect defects on the surface of transparent objects. When no defect is present, the bright-field image is bright and the dark-field image is dark. But, when the object has a defect, the dark-field image presents an increased brightness. However, this method of inspecting transparent objects only detects defects on a surface layer.
One prior art approach is disclosed in EP 902 275, which suggests an imaging apparatus and process for inspecting an object wrapped in a transparent or semi-transparent material, such as a cigarette package wrapped in a polymeric film. Incident light is directed upon the object such that the light enters the wrapper and the wrapper acts as a waveguide. The light escapes from the poly film wrap at edges and folds where it reaches a reflective boundary. The escaped light is captured by a camera and is thereafter sent to an imaging processor. Thus, overwrap defects, such as misplacement, misfolding, tearing, wrinkling or other defects, will be detected.
However, this prior art approach only detects defects in a wrapping layer. Defects on the package itself must be separately inspected at a separate stage requiring more than one inspection system, manual or automatic.
Therefore, there is a need for a system and a method for the detection of defects on an object comprising at least two layers, where at least the first layer consists of a transparent or semi-transparent material.